Micromachined devices fabricated by complementary metal-oxide-semiconductor (CMOS)-compatible fabrication processes are attractive because of the ability to integrate high-performance on-chip signal conditioning circuits with sensing elements, expected multivendor accessibility and short design cycle times. Currently, most CMOS-compatible micromachining processes are polysilicon or polycrystalline silicon/germanium surface micromachining process based which use silicon dioxide as the sacrificial material and typically involve a wet etch step for releasing the micromechanical structures. Even though HF vapor can be used for release, protection of integrated circuits and sticking problems during release still remain major concerns.
Wiring on a single polysilicon microstructure is constrained to one electrode for each continuous microstructure which limits the design flexibility for electrostatic actuators and capacitive sensors. Moreover, the relatively large parasitic capacitance in polysilicon processes degrades performance of capacitive sensor designs. For example, a 50-finger comb drive with 30 μm overlap in the MUMP's polysilicon process has about 28 fF of sensing capacitance. The parasitic capacitance is 13 fF due to the fingers alone, 14 aF/μm from interconnect and 1.1 pF for a standard 78 μm by 78 μm square bond pad. Bond-wire or solder-bump connection to external electronics contributes additional parasitic capacitance.
Miniature three-axis accelerometers are often required in automobiles, navigation systems and for some medical applications, such for use with hemiplegic patients. There are two types of micromachining processes, surface micromachining and bulk micromachining. Most existing micromachined accelerometers are uni-axial or dual-axial and fabricated using surface micromachining processes.
In general, bulk micromachining creates large proof mass and is suitable for making z-axis accelerometers with capacitive parallel plates or piezoresistive beams. However, typically no CMOS circuitry is integrated on the sensor chip. Surface micromachining, on the other hand, can be compatible with CMOS processes and is suitable for fabricating lateral-axis accelerometers with capacitive interdigitated comb fingers. By assembling and orienting orthogonally two or three separate accelerometers, tri-axial acceleration sensing systems can be obtained, but both the package size and cost is high.
Some 3-axis accelerometers have been reported. Among them, bulk micromachined 3-axis accelerometers generally have large mass, but require wafer bonding, wet etching and two-side alignment. Surface micromachined tri-axial accelerometers can have integrated interface circuitry, but have small mass.
Lemkin et al. discloses a surface micromachined 3-axis accelerometer [Lemkin, et al. “A 3-Axis Force Balanced Accelerometer Using a Single Proof-Mass” Transducers '97, 1997 International Conference on Solid-State Sensors and Actuators, Chicago, Jun. 16-19, 1997, pgs. 1185-1188]. Lerikin's accelerometer uses single-crystal silicon as the substrate material, but the sensor microstructures are made of thin-film polysilicon. The x- and y-axis sensing elements disclosed by Lemkin are comb fingers, while the z-axis sensing capacitance is formed as a parallel plate pair between the proof mass and a ground polysilicon layer on the substrate. Thus, a separate fixed capacitor is used to realize a differential capacitive bridge for z-axis sensing. Significantly, the inherent large parasitic capacitance greatly reduces the obtainable signal-to-noise ratio. The residual stress of the thin-film materials also limits the size of the proof mass which limits the obtainable resolution of the Lemkin's accelerometer.